Hydraulic clutches are well known in general, and can be found in many systems and devices. In one implementation, a set (plurality) of hydraulic clutches are used to facilitate shifting of a transmission between differing input/output gear ratios or ratio ranges. More generally, a transmission typically includes an input shaft, an output shaft, and a collection of interrelated gear elements, such as in a planetary arrangement or otherwise, usable to selectively couple the input and output shafts. The clutches may be used to select gear ratios in a discrete transmission, and to select gear ratio ranges in a continuous transmission. Both types of coupling will be referred to herein as “ratios.”
The selection of a gear ratio at the output shaft is executed via one or more clutches that affect the rotations and/or interrelationships of the gear elements. The clutches are typically hydraulically actuated to engage band or disk torque transfer elements. Shifting from one gear ratio to another normally involves releasing or disengaging an off-going clutch or clutches associated with the current gear ratio and applying or engaging an on-coming clutch or clutches associated with the desired gear ratio. By way of example, although many different clutch arrangements are possible within such transmissions, one possible arrangement is a two-clutch shifting transmission. In this arrangement, two clutches are required to hold a specific gear in said transmission. Typically, this entails a primary clutch, often a rotating clutch element, which is retained for an upcoming gear, and a secondary clutch that is disengaged in order to shift into the upcoming gear. The secondary clutch for this shift condition is referred to in the art as the off-going clutch. This clutch is replaced by a new clutch, the “on-coming” clutch, required to actuate the transmission into the new gear. In other words, a shift is executed by deactivating a single “off-going” clutch, activating a single “on-coming” clutch, and holding a third clutch for both the old and new gears. In other arrangements, multiple on-coming and\or off-going clutches are employed, increasing the complexity and criticality of clutch actuation timing.
Each hydraulic clutch is typically driven via an electrically controlled solenoid valve. Such solenoid valves are electrically modulated to control hydraulic fluid pressure to the clutch and hence to control the clutch piston movement during the clutch fill phase.
The phasing of the on-coming and off-going clutch element can have a substantial impact on the perceived shift quality. For example, if the off-going clutch disengages prematurely, the engine speed may surge briefly before the on-coming clutch, still in the fill phase, possesses sufficient torque capacity. Furthermore, if the on-coming clutch fills prematurely, the clutch element has sufficient torque capacity before the off-going clutch is ready to commence torque transfer. This can lead to a three-way clutch tie up which is detrimental to the transmission's useful life in a mild case, and often results in mechanical damage to the transmission in an extreme case. Conversely, in the event of a late clutch fill, the off-going clutch hands off torque to the on-coming clutch before the on-coming clutch has sufficient torque capacity, and the transmission slips as the on-coming clutch does not have sufficient time to lock with adequate torque capacity to hold the specific gear in question. The end result is a slip phenomenon in the clutch discs, also an undesirable event as this tends to produce high clutch energies resulting from excessive heat generation produced by the higher clutch relative velocities of the rotating clutch discs. In addition to creating an unpleasant user experience, badly timed shifting will over time, impact the efficiency and service life of the transmission. To this end, it is desirable to actuate the clutches with precision such that a smooth shift occurs throughout the entire operating speed range of the transmission during its entire useful life.
Known methods for calibrating transmission clutches tend to be empirical rather than contemporaneous. In other words, the behavior of the clutch may be observed at some point, and conclusions may be drawn as to how the clutch reacts to hydraulic flow. These observations are then used to periodically “calibrate” the clutch. However, the condition and operating environment of a clutch can change substantially between calibration intervals, resulting in a degradation of shift quality.
Although the resolution of deficiencies, noted or otherwise, of the prior art has been found by the inventors to be desirable, such resolution is not a critical or essential limitation of the disclosed principles. Moreover, this background section is presented as a convenience to the reader who may not be of skill in this art. However, it will be appreciated that this section is too brief to attempt to accurately and completely survey the prior art. The preceding background description is thus a simplified and anecdotal narrative and is not intended to replace printed references in the art. To the extent an inconsistency or omission between the demonstrated state of the printed art and the foregoing narrative exists, the foregoing narrative is not intended to cure such inconsistency or omission. Rather, applicants would defer to the demonstrated state of the printed art.